Ensure the conformance build of objc code is using the Mac OS X SDK

JSON handling for Any

This required a rework of the tokenizer to allow for a "replaying" tokenizer, basically in case the @type value comes after the data itself. This rework is nice in some ways (all the pushback and object depth logic in one place) but is a little fragile in terms of token push-back when using the replay tokenizer. It'll be fine for the scenario we need it for, but we should be careful...

Add region tags for protocol buffers tutorials.

Since these tags might be confusing, added a note that these are not
part of the normal protocol buffers syntax.  I also linked to the main
tutorials page that uses these examples
https://developers.google.com/protocol-buffers/docs/tutorials so that
anyone who arrived here without going through that info first can get
more explanation if they want.

Add a Go language example.

Fix some narrowing implicit casts [NFC]

Fix minor typo in LazyField javadoc

This follows the other examples so that it can be used as a tutorial,
such as the ones at:
  https://developers.google.com/protocol-buffers/docs/tutorials

Even though Go generally does not use Makefiles, I added targets for the
Go examples to be consistent with the other languages.

Edit:

Fix Travis run. Change to use $HOME instead of ~. Add protoc to path.
GOPATH entry cannot start with shell metacharacter '~': "~/gocode"

Edit(2):

Fix Go code style to address comments.

"equals".equals("euqals") == false

Adds support for building protobuf with emscripten

Adds support for buliding protobuf with emscripten; we just needed the proper platform define used in the right place.

Update GPBProtocolBuffers.h

Set the stream limit to the length of the data.

- Mark all conformance tests as now passing.

Add support for the conformance test for objc when run on OS X

Tidying up reflection further

Generated code changes for previous commit (basically InternalBuildGeneratedFileFrom => FromGeneratedCode)

There are corner cases where MessageDescriptor.{ClrType,Parser} will return null, and these are now documented. However, normally they *should* be implemented, even for descriptors of for dynamic messages. Ditto FieldDescriptor.Accessor.
We'll still need a fair amount of work to implement dynamic messages, but this change means that the public API will be remain intact.

Additionally, this change starts making use of C# 6 features in the files that it touches. This is far from exhaustive, and later PRs will have more.

Generated code changes coming in the next commit.

Added the type registry in advance of implementing Any support.

Biting off just this bit first as I don't need the changes from a previous PR for this part.

Fixed a bug in CSharp SampleUsage.

Rename "umbrella" to "reflection" consistently.

add headers

This changes csharp_names.h, which will require a corresponding change in GRPC.

Parser property in MessageDescriptor

Generated code coming in next commit - in a subsequent PR I want to do a bit of renaming and redocumenting around this, in anticipation of DynamicMessage.

Change the way the "umbrella class" (descriptors) is named/namespaced